Underwater Photography: Understanding Aperture, F-stops, and Depth of Field
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In the world of underwater photography, mastering the art of aperture is crucial. This essential element of photography controls the depth of field, the bokeh (background blur), and the amount of light that enters the lens.
The aperture, an opening within the lens, can be made larger or smaller, and this is referred to as an F-stop. A small F-stop (e.g., - 2.8) represents a large aperture, allowing more light to pass through, while a larger F-stop (e.g., - F22) indicates a smaller aperture, letting in less light. As the aperture is made smaller, the F-stop increases in number (e.g., F8, F11, F16).
Aperture significantly impacts the performance of lenses, particularly zoom lenses and non-prime lenses. Lenses often perform best when stopped down 1 or 2 stops. For instance, a Vase Tunicate was photographed at F18 with a 60mm lens in Raja Ampat, ensuring that the entire vase remained in focus without causing diffraction to ruin tiny details.
However, using too small an aperture can pose challenges underwater. Smaller apertures on a dSLR, around F20 & greater, will result in loss of sharpness (Diffraction) at 100% magnification. This was evident in a 100% crop of a photo taken at F29, where diffraction caused a noticeable loss of sharpness.
On the other hand, a large aperture such as F2.8 will heavily blur the background, as seen in a goby photographed at F4, where only the eyes were in focus, and the background was blurred very smoothly.
The depth of field (DOF) is an important concept. It refers to the area of a photo that is in focus. For example, when photographing a nudibranch at F25 in Anilao, the rhinophores and gills were in focus, but the background became distracting due to the shallow depth of field.
The effects of diffraction are normally not noticed when posting photos "on the web", but you will notice the difference at 100% magnification. Interestingly, cameras with smaller sensors, such as compact cameras with sensors smaller than APS-C or Micro Four Thirds sensors, typically have a greater depth of field (more area in focus) than DSLRs with larger sensors because smaller sensors inherently increase depth of field at the same aperture and framing.
When taking a photo underwater, consider how much depth of field you want, if you want the background blurred or in focus, and if you are worried about losing image sharpness to diffraction. Backing away from your subject gives you more depth of field, while getting closer to your subject will give you less depth of field.
For the best results at getting a blurred background, shoot wide open at F2.8, zoom all the way out, and get close to the subject. Wide-angle (WA) lenses have large depths of fields, while macro photos taken at 1:1 magnification will have very small depths of field.
Remember, when photographing the front of a nudibranch, you have three choices when choosing your f-stop: blur the gills in the background (large aperture between F4 and F9), slightly blur the rear of the nudibranch (medium aperture between F10 and F16), or get as much of the subject in focus as possible (small aperture between F18 and F29).
For those seeking further reading, the articles "Understanding ISO in Underwater Photography" and "Using Shallow Depth of Field Underwater" provide valuable insights into these topics.
